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          <h1 class="post-title" itemprop="name headline">阿里二面：NIO为什么不适合文件上传场景、如何优雅解决</h1>
        

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        <div id="vip-container"><p>见字如面,我是威哥,一个从普通二本院校毕业,从未曾接触分布式、微服务、高并发到通过<strong>技术分享</strong>实现职场蜕变，成长为RocketMQ社区优秀布道师、大厂资深架构师，出版《RocketMQ技术内幕》一书，在CSDN中记录了我的成长历程，欢迎大家关注我，随时可私信我，一起交流进步。</p>
<p>该系列已分别介绍了服务端、客户端的启动流程，本文将重点剖析Netty是如何封装NIO的读事件。</p>
<blockquote>
<p>温馨提示：本文虽然是源码分析，但强烈建议精读，因为根据源码阐述其背后的设计哲学，也用黑体进行了标注，请特别留意。</p>
</blockquote>
<p>在阅读本篇文章之前，请稍微思考如下几个问题：</p>
<ul>
<li>NIO为什么不适合文件上传等场景</li>
<li>NIO如何避免一个超大数据传送的连接对其他请求的影响</li>
<li>NIO如何处理半关闭</li>
</ul>
<h2 id="１、读事件概述"><a href="#１、读事件概述" class="headerlink" title="１、读事件概述"></a>１、读事件概述</h2><p>关于Read事件在SocketChannel与ServerSocketChannel所对应的操作不一样，在SocketChannel中，则对应数据读，而在ServerSocketChannel中则被被封装成接受客户端的连接请求。</p>
<p>NIO read事件入口在NioEventLoop的processSelectedKey方法，截图如下：<br><img src="https://img-blog.csdnimg.cn/20210313225717887.png#pic_center" alt="在这里插入图片描述"><br>其核心入口为UnSafe的read方法，关于UnSafe的类层次结构如下图所示：<br><img src="https://img-blog.csdnimg.cn/20210313230025369.png#pic_center" alt="在这里插入图片描述"></p>
<ul>
<li>AbstractNioByteChannel$NioByteUnsafe#read<br>SocketChannel对应的读事件处理流程，即IO读的处理实现。</li>
<li>AbstractNioMessageChannel$NioMessageUnsafe#read<br>ServerSocketChannle对应的读事件处理流程。</li>
</ul>
<p>接下来将分别介绍这两个流程。</p>
<h2 id="2、IO读事件从处理流程"><a href="#2、IO读事件从处理流程" class="headerlink" title="2、IO读事件从处理流程"></a>2、IO读事件从处理流程</h2><p>IO读事件由AbstractNioByteChannel内部类AbstractNioUnsafe的read方法实现，接下来重点剖析该方法，从中窥探Netty对IO读事件的处理。<br><img src="https://img-blog.csdnimg.cn/20210313233530876.png#pic_center" alt="在这里插入图片描述"><br>Step1：如果没有开启自动注册读事件，在每一次读时间处理过后会取消读事件，默认为自动注册。</p>
<blockquote>
<p>温馨提示：如果通道不注册读事件，将无法从通道中读取数据，即无法处理请求或接受响应。</p>
</blockquote>
<p>如果没有开启自动读事件，需要应用程序在需要的时候手动调用通道的read方法。</p>
<p>取消读事件，Netty基于NIO给出了非常标准的实现，基本可以当场模板代码使用：<br><img src="https://img-blog.csdnimg.cn/20210313235646288.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70#pic_center" alt="在这里插入图片描述"><br>其实现关键点：首先判断键值对是否有效，然后通过位运算进行取消注册。<br><img src="https://img-blog.csdnimg.cn/20210314000046309.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70#pic_center" alt="在这里插入图片描述"><br>Step2：创建接受缓存区内存分配器，这里有两个关键点：</p>
<ul>
<li>maxMessagesPerRead<br>每一个通道在一次读事件处理过程中最多可以调用底层Socket进行读取的次数，默认为16次，<strong>这里的设计哲学是避免一个通道需要读取太多的数据，从而影响其他通道的数据读，因为在一个事件选择器中多个通道的读事件是串行执行的</strong>。</li>
<li>RecvByteBufAllocator<br>接受缓冲区的内存分配策略，分为分配固定大小(不够时扩容)、动态变化(根据历史分配的大小，动态条件合适的内存大小)，<strong>这里主要的设计哲学是合理利用内存，并减少扩容，提高内存的分配效率与使用效率</strong>。</li>
</ul>
<p><img src="https://img-blog.csdnimg.cn/20210314001303653.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70#pic_center" alt="在这里插入图片描述"><br>Step3：循环处理读事件，最多处理maxMessagePerRead。接下来探讨一下单次读事件的处理流程。<br><img src="https://img-blog.csdnimg.cn/20210314125459781.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70#pic_center" alt="在这里插入图片描述"><br>Step4：进行一次IO读处理，其处理有如下几个关键点：</p>
<ul>
<li>首先分配一个ByteBuf，俗称接收缓存区，用来存放从网络中读取的内容。</li>
<li>获取一下分配到的累积缓存区可写的字节数，<strong>这个后面有妙用</strong>。</li>
<li>调用底层网络读API从网卡中读取数据，NIO的读取实现如下所示：<br><img src="https://img-blog.csdnimg.cn/20210314125905909.png#pic_center" alt="在这里插入图片描述"><br>即调用NIO中的SocketChannel进行读数据，其返回参数表示这次从网卡中读取到的字节数。<strong>如果读取到的字节少于0，则表示对端通道已关闭，己端也需要进行相应的处理，例如关闭通道</strong>。</li>
<li>读到一批数据后，会通过事件传播机制向事件链中传播channelRead事件，触发后续对该批数据的处理。</li>
</ul>
<p><img src="https://img-blog.csdnimg.cn/20210314130347920.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70#pic_center" alt="在这里插入图片描述"><br>Step5：判断该通道是否需要继续读，其基本依据如下：</p>
<ul>
<li>如果未开启自动注册读事件，读完一次之后将不再继续读取。</li>
<li>如果本次读取到的字节数小于接收缓存区，说明此刻网卡中没有可读数据，等下一次读事件触发再继续读。</li>
</ul>
<p><img src="https://img-blog.csdnimg.cn/202103141344362.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70#pic_center" alt="在这里插入图片描述"><br>Step6：一次或多次读操作结束后，会触发一次读完成事件，向整个事件链传播。</p>
<p>整个网络读处理流程就介绍到这了。</p>
<h2 id="3、接受连接处理流程"><a href="#3、接受连接处理流程" class="headerlink" title="3、接受连接处理流程"></a>3、接受连接处理流程</h2><p>在Netty中，服务端接收客户端的连接请求(OP_ACCEPT)，被封装在channelRead 事件中，其代码入口为：AbstractNioMessageChannel　的内部类NioMessageUnsafe的read方法。<br><img src="https://img-blog.csdnimg.cn/20210314135118821.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70#pic_center" alt="在这里插入图片描述"><br>其大概的实现要点在前面已经介绍，这里主要看一下NioServerSocketChannel的doReadMessage。<br><img src="https://img-blog.csdnimg.cn/20210314135339382.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70#pic_center" alt="在这里插入图片描述"><br>通过使用底层的NIO接受一个连接，并获取NioSocketChannel对象。然后继续该对象向下传播channelRead事件，在后续的处理器中对该对象进行操作，例如将其注册读事件，从而触发网络的读操作，关于NioSocketChannel如何绑定读事件、注册业务相关的事件监听器机制已经在<a target="_blank" rel="noopener" href="https://blog.csdn.net/prestigeding/article/details/113873810?spm=1001.2014.3001.5501">Netty进阶：手把手教你如何编写一个NIO服务端</a>中详细介绍，本文就不再重复。</p>
<p>好了，本文就介绍到这里了，想必对开头部门提出的问题有了自己的答案了吧，<strong>您的一键三连是对我最大的鼓励</strong>，当然可以加<strong>笔者微信：dingwpmz</strong>，备注CSDN，共同交流探讨。</p>
<hr>
<p>为了方便大家学习Netty，笔者将RocketMQ的网络通信模块抽取出一个通用的Netty开发框架，大家可以从github上下载，堪称Netty界最强Hello World。<br><img src="https://img-blog.csdnimg.cn/20210314140644894.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70#pic_center" alt="在这里插入图片描述"><br>下载链接：<a target="_blank" rel="noopener" href="https://github.com/dingwpmz/netty-learning">Netty通用开发框架github地址</a></p>
<p>分享笔者一个硬核的RocketMQ电子书：<br><img src="https://img-blog.csdnimg.cn/2021032212002741.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70#pic_center" alt="在这里插入图片描述"></p>
<p>获取方式：扫描如下二维码，关注【中间件兴趣圈】，回复RMQPDF即可获取。</p>
<p><img src="https://img-blog.csdnimg.cn/20190314214003962.jpg?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70#pic_center" alt="在这里插入图片描述"></p>
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